Do you leave yours in when putting away your firearm?  Good idea, bad idea.  This is for firearms that are not being used for self defense or those that should be in a state of readiness. 

I don't use snap caps but I don't see how it could hurt to leave them in. That being said, you'd probably just eject them the next time you check your firearm putting more wear on it.

I've got an O/U Beretta shotgun.  The only way to decock it is to pull the trigger, but O/Us are notorious for breaking firing pins that way.  So I do leave homemade snap caps in that gun during storage.

Everything else I can either decock or dry fire safely without a cap.  I even pull the trigger on my semi-auto .22s to decock them for storage.  Yeah, I know, never dry fire a rimfire... I call BS.  I accidentally dry fire them all teh time, simply not being ably to count to 5 or to 10 at the range.  That extra once in a while isn't going to matter.  Been doing it for decades, and never once had a firing pin problem or a breech face peening problem.  But I digress.

Most snap caps are either plastic or are made with the same materials as ammo.  They really can't hurt the gun in any way.  I have had a case where the residual solvent in the chamber eventually softened and deformed a plastic snap cap, so I avoid those now.